A few years ago I was really into modding out my laptops -replacing CPU's, swapping GPU's, replacing motherboards so that I got the newer revision, etc...
What I can tell you from this experience is that what you wan to do is try to find an example of your laptop on the web equipped with the CPU you want to swap. Unless Dell specifically released a N5030 with a quad core, it won't work. There's a lot of other things to consider, like CPU TDP, chipset, and BIOS limitations that can get in the way of a clean swap.
With the money you'd spend on a used QC on eBay, I'd say you're better off buying a used laptop with a first or second generation i-series CPU. Laptops equipped with those are no longer that expensive on the used market, and you'll get WAY better performance versus upgrading that N5030.
Gigabyte X99 UD3P - i7-5820k 4.3GHz - 16GB DDR4 2400 HyperX - MSI R9 390 8GB - 500GB m.2 Samsung 850 Evo -
750 Watt eVGA Supernova G2 - CM Hyper 212 Evo - Antec P60 - Win 10 Pro x64 - LG 34" Ultrawide w/Freesync
2011 Macbook Pro
2.0GHz i7 2635QM - 16GB DDR3 1600 (2 x 8GB) - 500B Samsung 850 Evo SSD - 15.6" matte LED - Radeon 6490m - OS X 10.11/Win10